A two-component developer composition comprising a toner and a carrier is frequently used as a developer for developing an electrostatic latent image in electrophotography, etc.
Various carriers for the two-component developer composition are known, typically including electrically conductive carriers, exemplified by iron oxide powder, and coated insulating carriers. The conductive carriers are excellent in solid reproducibility but poor in fine line reproducibility. Besides, toner particles are fused and adhered onto the surface of the carrier, resulting in considerable reduction of chargeability. On the other hand, coated type insulating carriers have poor solid reproducibility, though excellent in durability and fine line reproducibility.
In order to eliminate these disadvantages, a small size carrier comprising a binder resin having dispersed therein magnetic fine particles, so-called a carrier for microtoning, has been proposed and put into practical use. Having a small true specific gravity, high insulating properties, and a small particle diameter, the carrier of this type is known to make a denser and more uniform magnetic brush than by the conventional carriers, and thereby to provide images of improved quality in density reproducibility and freedom from noise, such as brush image.
It is known to use various inorganic or organic fine particles as an external additive for toners in order to improve fluidity, anti-caking properties, fixability, chargeability, cleaning properties, etc. to thereby broaden process suitability of toners. Known additives for such purposes include silica, titanium oxide, aluminum oxide, and tin oxide.
Developer compositions comprising such a toner and the above-mentioned magnetic powder-dispersed type carrier have also been proposed. For example, JP-A-60-136775 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a developer comprising a magnetic powder-dispersed type carrier and a toner having adhered thereto titaniumoxide/silica fine powder, and JP-A-61-9661 discloses a developer comprising a magnetic powder-dispersed type carrier and a toner having adhered thereto silica fine powder.
On the other hand, a copying machine equipped with an automatic toner concentration controller has recently been adopted, in which a toner is automatically supplied to a developer according to a monitored reduction in image density. However, such a copying machine has turned out to have the following disadvantage when combined with the above-mentioned developer compositions comprising a magnetic powder-dispersed type carrier and a toner having adhered thereto inorganic oxide fine particles. That is, the charge exchanging properties between the toner and the carrier, though satisfactory at the initial stage of copying and after producing 10,000 copies, are deteriorated in the stage intermediate therebetween, i.e., in the relatively initial stage of copying around 5,000 copies, resulting in background stains.
It is assumed that such background stains in the relatively initial stage of running arise for the following reasons. A developer composition containing the magnetic powder-dispersed type carrier has poor fluidity as a whole due to the fact that the carrier has low magnetic properties, it is used as having a relatively small diameter, it has a smaller specific gravity than general carriers, and it is amorphous. Further, since the carrier exhibits strong binding properties to the previously or initially mixed toner particles, the probability of contact between the carrier and a fresh toner which is supplied after the previously or initially mixed toner particles are consumed to some extent is considerably lessened. As a result, the frictional contact between the carrier and the fresh toner is so limited that the charge exchanging properties therebetween is markedly reduced. Therefore, in the copying machine equipped with an automatic toner concentration controller, when the charge quantity of toner particles abnormally increases to cause a reduction in image density, and, accordingly, the developer is automatically replenished with a fresh toner, the charge exchanging properties between the fresh toner and the carrier are so low that frictional electrification among toner particles is accelerated, resulting in production of toner particles of opposite polarity or of insufficient charge quantity which lead to background stains of images.
The fact that no background stain occurs at the initial stage of copying and after copying for a long period of time appears to be accounted for as follows. At the beginning of copying, the charge quantity of the toner is still small, causing no problem. After copying for a long period of time, the inorganic oxide fine powder adhered on the surface of the toner particles is released and transferred to the interface with the carrier to serve as a fluidity aid which increases chances for the carrier and the toner to undergo frictional contact with each other and, at the same time, prevents the toner from overcharging, thereby causing no problem.
The above-described disadvantage associated with a copying machine equipped with an automatic toner concentration controller might be eliminated by adding inorganic oxide fine particles to the toner in excess so as to improve charge exchanging properties in the initial stage of running. However, such a means reduces an absolute charge quantity, eventually leading to considerable impairment of long-term reliability.